Add user settings to delegate control

[sm13]

Right now the query, and in general all actions are hardcoded, for instance:

    let complete_query = QueryBuilder::default()
        .set_state("Unread")
        .set_favorite(0)
        //.set_tag("rust")
        .set_content_type("Article")
        .set_sort("Newest")
        .set_detail_type("Complete")
        //.set_search("learn")
        //.set_domain(".com")
        .set_since(since)
        .set_count(10)
        .set_offset(0)
        .set_total(1)
        .build();

However, some of these could be read from a user-defined settings file so that the query can be adapted to different needs or requirements. For instance, a new ./local/share/repocket/settings.json could contain:

{
    "query" : 
    {
        "tags": ["rust", "remarkable"],
        "only_favorites": 0,
        "max": 5
    }
}

When this is implemented, a set of default settings should match the current behavior, so as not to change it.